1. Field of the Invention
The present invention generally relates to reducing noise and interference between electronic communication devices that utilize the same transmission media for communications. More particularly, the invention relates to estimating or modeling transmission media channel characteristics in a networking or other communications system by creating a model of the impulse response that can be processed to reduce noise and interference between network or other communications devices coupled to the same transmission media.
2. Description of the Related Art
Signal processing techniques using filters, such as echo cancellers and channel equalizers, are important for communications system operation. Echoes can interfere with data transmission and degrade communications system performance. Echoes are transmission channel impairments that result from signals reflected at points in the transmission media where impedances are not matched. Echoes can be produced in hybrids in telephone lines or in modems. A hybrid is a special circuit disposed between two-wire circuits and four-wire circuits in telephone line connections. When used in a modem, a hybrid permits the transmitter and the receiver to be connected at the same time to the telephone line.
When a transmitted signal flows through a network, such as the telephone network, an echo canceller can model the echo path from the echo that is created. Typically, the echo canceller is a type of tapped-delay-line, an adaptive finite impulse response (FIR) filter. It is adaptive because the channel to be modeled is unknown and can change during the course of communications. Echo cancellers can be located within or at the ends of a communications network. They operate to cancel echoes by estimating a replica of the real or true echo and then subtracting that estimate from signals containing the real echo. Typically, an incoming or received signal is a combination of the real echo and the desired true or real signal that has been impaired by noise and other channel distortions. The received signal is input to summation circuitry, where the echo replica is subtracted from the real echo imbedded with the true signal, leaving the desired signal with impairments other than echoes to be passed. Echo cancellers can be implemented as passband or baseband circuits.
Inherent in an echo is a time delay. Accurate detection of the echo time delay must be made for proper echo cancellation. This is accomplished by use of training signals, which are sent out during a training period by the modem to detect the presence, characteristics, and delay of an echo. The training of an echo canceller involves replicating the echoes, as indicated above. For far-end echoes, the modem must be able to cancel out the effects of any transmission media channel impairments that have affected the echo as it passed through the communications system. Such impairments include noise, frequency translation, envelope distortion, and attenuation distortion. For this purpose, channel equalizers are employed, and the echo canceller's adaptive tapped delay line and its tap update registers must be accurate. The tap delay line must be long enough to span the echo, and the modem's tap update register must be precise enough to properly discriminate for echo cancellation. The modem must also produce effective equalization signals in the presence of channel impairments. Other types of modems that use frequency division multiplexing rather than echo cancellation may not be as affected by such impairments.
The process of replicating the echo involves setting initial coefficients or tap weights of the echo canceller and using an error signal to update or adjust the echo canceller coefficients, as will be appreciated by those skilled in the art. As training occurs, the echo canceller filter coefficients are updated. The better the update, the more accurately the true echo will be replicated for proper cancellation. Updating changes (i.e., reduces) the error signal so that the echo canceller coefficients converge rapidly on the proper values to replicate the true echo value. The idea is to use as few taps as possible to accurately estimate the true echo. The length of the channel's impulse response in the time domain determines the number of coefficients (or tap weights). More specifically, the number of coefficients must be enough to span the length of the channel's impulse response. The coefficients are generated at the baud rate of the communications system.
The estimate of the echo path response has to be accurate enough before useful data transmission can start. With the typical least-mean-square (LMS) technique used during initialization training, data are sent out and an LMS adaptation algorithm is run to obtain the proper coefficients for the echo canceller. However, because of the slow convergence speed of the LMS technique, a long training period of a few thousands of data symbol intervals is usually required to obtain a reasonable level of echo cancellation. In LMS, the training sequences are used in a correlation process, and the technique requires higher computational capabilities. More information can be obtained on the LMS technique in S. B. Weinstein, A Passband Data-Driven Echo Canceller for Full-Duplex Transmission on Two-Wire Circuits, IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. Com-25 (July 1977), which is hereby incorporated by reference in its entirety. Another technique is to use discrete Fourier transform (DFT). Other techniques for echo cancellation of near-end and far-end echoes include sending a special periodic training sequence and correlating a portion of the sequence with real echo samples. This technique is disclosed, for example, in Guozhu Long and Fuyun Ling, Fast Initialization of Data-Driven Nyquist N-Band Echo Cancellers, IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. 41, No. 6 (June 1993), which is hereby incorporated by reference in its entirety.
Further information on echo cancellers, modems, and hybrids can be found in Jack Douglas, V.32 Modems are Breaking Through the Echo Barrier, DATA COMMUNICATIONS (April 1988), Steven E. Turner, Echo Cancellation for High-Speed Dial-Up Applications: Part I, TELECOMMUNICATIONS (January 1988), Steven E. Turner, Echo Cancellation for High-Speed Dial-Up Applications: Part II, TELECOMMUNICATIONS (February 1988), Jin-Der Wang and Jean-Jacques Werner, Performance Analysis of an Echo-Cancellation Arrangement that Compensates for Frequency Offset in the Far Echo, IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. 36, No. 3 (March 1998), and in Fuyun Ling and Guozhu Long, Correlation-Based Fast Training of Data-Driven Nyquist N-Band Echo Cancellers, IEEE INT'L CONFERENCE ON CCOMMUNICATIONS (ICC '90 and SUPERCOMM/ICC '90) vol. 4, (April 1990), which are hereby incorporated by reference herein in their entireties.
In communications systems, for example, for voice band data-modems that employ channel equalizers to remove channel impairments other than echoes, it is important that the initial training be fast as well. The equalizer is a filter like the echo canceller and also has coefficients or taps that must be estimated and updated (i.e., properly weighted) based on characterization of the transmission media channel. The equalizer must be trained to compensate for phase, amplitude, and distortion impairments imposed by the transmission channel.
Sequences of signals are used for training both echo cancellers and channel equalizers. Such sequences are wide-band pseudo random periodic time domain signals. For equalizers, the period is equal to the length of the equalizer delay line. A sequence that has complex symbols having constant amplitude facilitates gain control, and if the spectrum of the training signal has lines of equal amplitude, then such combination of characteristics are termed constant amplitude zero auto correlation (CAZAC) sequences. Such sequences can be designed for the length of the particular equalizer. Although the equalizer must be trained much like the echo canceller, its role is to remove transmission media channel impairments other than echoes, as discussed above. Typically, the channel equalizer follows the echo canceller in the received signal path for processing the received signal. Further information on equalizers can be found in Pierre R. Chevillat, Dietrich Maiwald, and Gottfried Ungerboeck, Rapid Trading of a Voiceband Data-Modem Receiver Employing an Equalizer with Fractional-T Spaced Coefficients, IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. Com-35, No. 9 (September 1987), which is hereby incorporated by reference herein in its entirety.